In recent years, a super luminescent diode (hereinafter referred to as “SLD”) is drawing attention.
The SLD differs from a semiconductor laser and a light emitting diode (LED). The semiconductor laser oscillates light having a high output power and a very narrow spectrum width with a low injection current through stimulated emission and further resonation of the light. The LED has a wide radiation angle using spontaneous emission light.
Specifically, in the SLD, there is such a feature that a high output power and a wide spectrum half-maximum width are obtained by employing a configuration of not resonating light even in a high current injection state while the light induced amplification is used.
Through use of those features, applications of the SLD have been expanded into various fields, such as a spectroscope, a length measuring instrument, a refractive-index distribution measuring apparatus, a tomography apparatus, and a light source for excitation.
As described above, in order to realize a particularly wide spectrum half-maximum width, in the SLD, high current injection is needed to operate a device compared to that of a semiconductor laser.
The characteristics of light emission of the SLD are described below with reference to FIGS. 2A to 2C.
FIG. 2A shows spectrum intensity in the case of using a single quantum well in a layer for emitting light (hereinafter abbreviated as “SQW”).
In FIG. 2A, a horizontal axis represents a wavelength, and a vertical axis represents spectrum intensity. Multiple spectrum waveforms correspond to different injection current level.
In FIG. 2A, when a spectrum waveform 201 at a time of lowest injection current is compared to a spectrum waveform 202 at a time of highest injection current, intensity of an emission wavelength at a high-order level (represented by an dotted arrow 203) increases and a spectrum half-maximum width is enlarged along with an increase in injection current.In this case, a spectrum shape changes in such a manner that a change on a long wavelength side is small, and intensity on a short wavelength side increases along with high injection current.
Next, FIGS. 2B and 2C show the case of using multi quantum wells in a layer for emitting light (hereinafter abbreviated as “MQW”).
FIG. 2B shows a band diagram on a conduction band side of two quantum wells having the same composition and different thicknesses. Black points represent electrons serving as carriers.
A quantum well 204 has a thickness smaller than that of a quantum well 205. Therefore, the quantum well 204 has a bandgap larger than that of the quantum well 205 owing to a quantum effect and is capable of emitting light having a short wavelength.
FIG. 2C shows gains corresponding to the respective quantum wells.
At a time of low injection current, electrons are accumulated in the quantum well 205 to emit light. When the injection current is increased, electrons are also accumulated in the quantum well 204 to emit light. At the same time, light is also emitted from an energy position higher than that of the quantum well 205, and hence the shape of a gain from the quantum well 205 changes from a form indicated by a solid line 207 to that indicated by a dotted line 208. Consequently, the intensity on a short wavelength side further increases (dotted arrow 209). As described above, irrespective of whether the SQW or the MQW is used, when the spectrum half-maximum width is enlarged, there occurs such a phenomenon that the intensity on a short wavelength side increases further compared to that on a long wavelength side, which is a feature of the SLD.
The light on a short wavelength side may cause the following problems in terms of use.
That is, light having a short wavelength has high energy, and hence may damage a measurement system or an object to be measured depending on the wavelength.
Further, an increase in intensity caused by an increase in injection current is significant. Therefore, depending on the injection current, a spectrum half-maximum width rather becomes narrower, i.e., a spectrum shape is greatly deviated from a Gaussian shape, which may cause noise during measurement.
In particular, in the case of using the SLD as a light source for fundus OCT (optical coherence tomography), when light having a wavelength of 790 nm or less enters an eyeball, the luminosity factor is enhanced to cause the contraction of a pupil. Therefore, it becomes difficult to perform correct measurement.
Conventionally, as a method of suppressing an output on a short wavelength side, Patent Literature 1 proposes a semiconductor laser element described below.
FIGS. 11A to 11D show gain shapes and spectrum shapes of semiconductor laser elements according to a conventional example and an example described in Patent Literature 1. FIGS. 11A and 11B show the conventional example, and FIGS. 11C and 11D show the example of Patent Literature 1. FIGS. 11A and 11C show gain spectra 41 and light absorption spectra 42, and FIGS. 11B and 11D show an oscillation spectrum 45 of the conventional example and an oscillation spectrum 46 of the example described in Patent Literature 1, respectively. Further, FIG. 12 is a band diagram showing one configuration of the vicinity of an active layer in the semiconductor laser element according to the example of Patent Literature 1.
The semiconductor laser element of Patent Literature 1 has a configuration in which a light absorbing layer 14 and a separation layer 15 are arranged between a barrier layer 32 and a cladding layer 13 in a Fabry-Perot laser.